In wireless and mobile communications systems, channel quality information (CQI) about an uplink channel is estimated based on training or pilot signals sent to a base transceiver station (BTS) by a mobile station (MS) or a customer premise equipment (CPE). CQI includes the carrier-to-interference-plus-noise ratio (CINR), beamforming weighting vectors for multiple antennas, Doppler frequency, Received Signal Strength Indication (RSSI), etc.
A downlink burst (from a BTS to a CPE) is a group of symbols carrying data and/or pilot signals. Because modulation, coding and transmission power assigned to a downlink burst are determined based on downlink CQI, it is essential that the downlink CQI should be available before a downlink burst is transmitted.
Because of the reciprocal nature of downlink and uplink channels in a wireless communications network employing time-division-duplex (TDD), downlink CQI (e.g. beamforming weighting vectors for a BTS equipped with multiple antennas) can be estimated using uplink training symbols.
FIG. 1 is a diagram illustrates the allocation of a sounding region for sending training symbols in a wireless communications network employing TDD. A TDD frame comprises a downlink sub-frame 102 and an uplink sub-frame 104. A sounding region 106, which is part of an uplink sub-frame 104, is allocated on demand. FIG. 1 shows three consecutive frames.
A region in an uplink sub-frame is identified as a dedicated sounding region where an MS sends uplink training symbols to a BTS. A dedicated sounding region can be allocated in one of the two ways. The first way is to allocate a sounding region periodically. More specifically, a sounding region is allocated every k frames to an MS or for a service flow, denoted as connection identification (CID). A frame is defined according to the design of a wireless communications network. The length of a frame should be small enough to guarantee that downlink CQI is available for each downlink burst transmission request. The second way is to allocate a sounding region on demand. When there is a request to send a downlink burst, a sounding region is allocated by the BTS.
Transmitting training symbols in a dedicated sounding region incurs overhead, which consumes valuable radio resources such as bandwidth, power, etc. Consequently, the utilization rate of the wireless communication network decreases.
As such, it is essential for a wireless communications network to obtain sufficient CQI information while minimizing the overhead of transmitting training symbols.